3.1 Trap Sector
3.1.5 Daily research logbook data
Despite covering an area of 13,000km2, only 20 – 30% of the waters of Shark Bay are exploited by commercial crab trap fishers, primarily in the oceanic waters north of Cape Inscription. Almost 90% of the fleet’s catch came from these northern grounds at a mean CPUE of 1.6 kg/potlift, compared with 8% from the Eastern Gulf (1.3 kg/potlift) and just 5% from the Western Gulf (1.2 kg/potlift). Fishing activity in the northern grounds was centered on two main areas: from Cape Peron north-west to Koks Island, and along the eastern coast from north of Carnarvon to Bush Bay up to 30km from land (Fig. 50). While catches were highest in specific areas, the mean annual catch rate was relatively consistent across all areas fished in the northern grounds, with only slightly higher catch rates north of Cape Peron, east of Koks Island, and 20km south west of Carnarvon (Fig. 50). Spatial distribution between male and female crabs was homogenous across these fishing areas, other than the ground northwest of Peron, which produced significant quantities of male crabs but fewer females (Fig. 51). It must be noted, however, that the areas north of Cape Inscription that can be accessed by trap fishers are restricted for parts of the year by the operations of the Shark Bay prawn and scallop fleets. The prawn fleet operates each year from April through October, while the scallop fleet commences around April and can fish from 3 weeks to 6 months depending on the strength of scallop stocks. During these periods, trap fishers stay out of the areas open to trawlers (Fig. 2) to avoid their traps fouling the trawlers’
nets (Fig. 52).
Figure 50.Spatial distribution of mean annual commercial trap crab catch (kg) and catch rate (kg/traplift) between 2003 and 2008 in the waters of Shark Bay north of Cape Inscription, derived from daily blue swimmer crab research logbook data.
Figure 51.Spatial distribution of mean annual commercial male and female trap catch (kg) between 2003 and 2008 in the waters of Shark Bay north of Cape Inscription, derived from daily blue swimmer crab research logbook data.
Figure 52.Spatial distribution of commercial trap effort () during different phases of the trawl fishing season in the waters of Shark Bay north of Cape Inscription between 2007 and 2010. Data is derived from voluntary Department of Fisheries Daily Research Logbooks submitted by the Shark Bay trap fleet. (///) denotes areas permanently closed to trawling; (///) denotes temporal trawl closures; denotes area actually fished by trawlers.
3.1.5.2 Trawl Catch
Records of crab landings for the trawl sector commenced in 1989 and these were via monthly CAES records. Low landings, between 3 and 12 t of crabs were recorded as retained catch for the first four years (1989 to 1992). From 1993 to 2001 inclusive the crab annual landings increased, ranging from 25 to 89 t with an average of 51 t. Reporting of byproduct landings by the trawl fleet using logbooks and validation through processor unloads commenced in 1997.
Historical logbooks did not record byproduct because the focus of recording was the target species. Between 1978 and 1996 the statutory reporting of byproduct was through monthly CAES records and the accuracy of the reported crab catches cannot be validated. Low annual crab catches were reported in these early years with reporting improving by the mid 1990's. It is likely there was under-reporting of byproduct landings, including crabs, between 1978 and 1996.
In 2002 there was a significant increase in the retention of crabs by the prawn boats. From 2002 to 2010 the total landings of crabs from the prawn boats were in excess of 100 t with an average annual catch of 190 t. (Table 4). The annual landings were approximately 250 t of crabs in 2007 and 2009 and peaking in 2010 at 338 t. Scallop boats retain a very small amount of crab and the annual total landings are in the order of 10 t.
In the years 1998 to 2005 inclusive most of the crabs were retained in the early part of the season, whereas from 2006 most crab numbers were retained in the middle of the season May to August (Fig.
10). This tends to fit the delayed prawn fishing strategy by opening the eastern part of the fishery later and rolling opening of areas where crabs are mostly abundant in the early part of the season.
Effort
The trawl fishery in Shark Bay commenced with 4 boats fishing a total of 2420 hrs in 1962. Fishing boat numbers increased and peaked at 35 in 1975. Fishing effort increased rapidly and peaked at 70,000 hours in 1981, but that was an exceptionally high year for total prawn landings and the nominal effort range was between 55,000 and 66,000 hours. The nominal effort remained stable around 60,000 hours until a reduction of boat numbers from 35 to 27 boats in 1990 occurred. The boats towed twin gear (16 fathoms headrope length). Fleet rationalisation to increase fleet efficiency commenced in 2005 and reduced boat numbers from 27 to 18 with those boats remaining towing quad gear (22 fathoms). Although boat numbers were reduced by 33% the total net headrope length was only reduced by 8%. Because of rising costs of fishing and a decline in prawn prices since the mid 1980s fishing effort has also decreased to gain efficiency and maintain profits. The comparable effort since 2005 is around 39,000 hours and the annual nights fished have reduced from a high of 226 to around 170.
Crab retention
Retention of crabs from individual prawn boats varies, with annual landings of blue swimmer crabs ranging from 3 to 28 t. From a total landed catch of 251 t in 2009, eight trawlers averaged 20 t, whereas the other ten boats averaged only 9 t. The total crab landing for the 2010 fishing season increased to 338 t (Table 4; Fig. 7), demonstrating the potential for higher trawl crab catches if all boats retained crabs at the higher retention rates.